Upgrade VS 2019 Professional to latest version 16.10.0. Open an existing SSDT project, double-click on any sql script file in the project, the file can't be opened in a tab window.
If the sql script defines a table, I can see the message: The design surface is loading. I can see the separator between design and script surfaces. If I drag the separator to resize the surfaces, the window gets refreshed and the script will open correct.
If the sql script defines a view or stored procedure, I only see blank tab window. i don't see any messages.
If I restart the VS2019, the previous opened scripts are working fine. But still the same problem whenever I want to open a script which is not opened before the restart.
In VS 2019 Community 16.10, I can open one SQL file at a time. So if you shut all your open SQL files, you should be able to get a single one open. Note: even uninstalling VS and starting with a clean install from the Microsoft site did not help.